They've tried to solve the blurring social contexts problem of facebook with something called 'aspects.' Aspects are user defined sets based on social context, eg 'work', 'school'. You can choose to make any activity on diaspora visible to certain aspects, and defining aspects is supposed to be pretty central to the diaspora interface—although I haven't ever used it so I can't really say for sure.

Facebook kind of solved this problem in the meantime by making facebook groups more intuitive. When diaspora was first announced, this social context issue was perceived as a bigger problem with facebook than it is now.